HIGH VERSATILITY MACHINE FOR THE ELECTROPHORETIC RECOATING OF TINPLATE LIDS

Abstract: A machine for the electrophoretic coating of thin-walled metal objects, such as tinplate lids, of the type comprising a containing tank (3) for the electrophoretic bath (4), a cathode immersed in the bath and an anode partially immersed in the bath and brought into contact with the objects to be coated, and a conveying line for advancing the objects immersed in the bath, the anode consisting of a vertical endless chain (50), the lower branch of which runs above the liquid surface of the electrophoretic bath, and from which there extend electrodes in the form of flexible appendices (55) to be partially immersed in said bath in order to make contact with the objects to be coated.

Description: HIGH VERSATILITY MACHINE FOR THE ELECTROPHORETIC RECOATING OF TINPLATE LIDS This invention relates in particular to the coating of tinplate lids for containers such as cans, and more generally to a machine for coating thin metal objects by an electrophoretic process. As is well known, tinplate lids are obtained by usual punching/drawing operations starting with a flat thin metal sheet. This is usually precoated, but if not the respective lids are coated after formation. In any event, after such operations at least the lid perimetral edge, which is turned towards its inner face to be subsequently clinched to the respective container, is without coating. The lids are sometimes subjected to a conversion operation after their formation. This conversion involves the formation of a preferential fracture line, plus possibly a small hole for fixing (by riveting) a tearing tab, such converted lids commonly being known as "easy opening type". Hence even in the case of lids formed from precoated sheet there is the problem of recoating those regions of the lids which have been bared either during their forming, or during their conversion or accidentally. A machine is specifically known, described in Italian patent application RE 91A00063 in the name of the present applicant, for the electrophoretic coating of lids. Said machine comprises a process tank containing an electrophoretic bath, and within which there is arranged at least one conveyor line on which the lids are placed flat with their turned edge facing upwards, and completely immersed in the bath. In the same bath there is also immersed, in addition to a cathode, an anode positioned parallel to said at least one conveyor line to preferably traverse at the same speed as this latter while resting on each individual lid for the entire period during which they traverse the process tank. Said conveyor line comprises in particular an endlessly extending horizontal conveyor such as a toothed belt or chain, comprising a plurality of equidistant projecting elements which urge the lids along two opposing longitudinal guides at an adjustable distance apart. The outward and return portions of said conveyor are arranged respectively above and below the base wall of the process tank, the former passing through the end walls of this latter by virtue of respective weir openings. A part of the electrophoretic bath discharges continuously via these latter into an underlying collection tank, from which it is again returned to the process tank. The positive electrode or anode consists according to the invention of a rectilinear length of wire or metal tape able to follow an endless path, its outward portion resting on an edge portion of the conveyed lids in such a manner as to define two points (in the case of wire) or lines (in the case of tape) of electrical contact with the cut periphery of the turned edge of the lid. This machine, further details of which can be obtained from the said patent application, suffers however from a drawback which considerably limits its use. Being immersed in the bath, the wire forming the positive electrode or anode becomes covered with a large quantity of coating, which is practically impossible to remove and soils the entire wire guide means in a manner which cannot be remedied. The same thing happens for the two wire deviation pulleys. The object of the present invention is to obviate the aforesaid drawback. This object is attained according to the invention by an anode consisting of an endless metal chain located entirely outside the bath and provided with vertical perpendicular appendices which along the lower part of the chain path touch the lids immersed in the bath. The characteristics and constructional merits of the invention will be apparent from the detailed description given hereinafter with reference to the accompanying figures, which illustrate a preferred embodiment of the invention by way of non-limiting example. Figures 1A and IB are two sectional elevations showing the upstream and downstream parts of the machine respectively. Figure 2 shows part of the section on the line II-II of Figure 1A, to an enlarged scale. Figure 3 is a section on the line III-III of Figure IB, to an enlarged scale. Figure 4 is an enlarged view of the part shown within the circle indicated by IV in Figure 2. Said figures, and in particular Figures 1A, IB and 3, show a casing 1 divided into various service compartments by suitable walls, and comprising a process chamber indicated by 2. In this latter there is provided a tank 3 for containing the electrophoretic bath 4, the tank being positioned between an overlying tank 5 for containing a cleaning liquid 6 (Figure 3), and an underlying tank 7 for collecting and recirculating the electrophoreti liquid falling from above. By way of example only, said electrophoretic liquid 4 consists of a solution, suspension or dispersion usually comprising from 1 to 5% of varnish or other suitable material, and from 99 to 95% of deionized water. As can be seen, the tank 3 receives the outwardly travelling branches of two conveyors 44 provided for advancing the lids 8. A chain 50 is provided in the form of a vertical endless loop such that its rectilinear outwardly travelling branch lies above the liquid surface of the bath contained in the tank 3, and its return branch is immersed in the bath of the tank 5 (Figures 1A, IB, 3). Perpendicular appendices 55 extend outwards from the chain 50 to form the electrodes to make contact with the lids to be coated. The conveyors 44 a d electrodes 55 can be provided in any number. The electrode 55 will also be known hereinafter as the anode (or positive pole of the bath 4), but this does not exclude the possibility of the electrode 55 acting as the cathode (or negative pole of the bath) if certain types of objects and/or varnishes or lacquers are present. The conveyor 44 consists of a toothed band (or toothed belt or chain) extending as a horizontal loop about two opposing pulleys 9 and 10. On the shafts 90, 100 on which these latter are fixed, there are keyed corresponding pairs of ring gears 91, 101 which act as transmission members for a conveyor 66 on which the lids 8 to be recoated arrive and a conveyor 77 on which the recoated lids 8 leave. Although not shown in detail, the pulleys 9, 10 and ring gears 91, 101 are suitably electrically insulated. As can be seen, said three conveyors 44, 66 and 77 are provided with respective projecting elements or pushers 40, 60 and 70 respectively, these being provided for advancing the lids 8 and being distributed at a constant pitch. This distribution pitch is particularly large, enabling said conveyors to handle lids 8 of considerably different diameters or side dimensions. The arriving conveyors 66 are connected to a convenient feed device, such as a conversion press followed by a pretreatment module, the departing conveyors 77 being connected to a convenient collecting and/or treatment device, such as a wash chamber followed by an oven for curing the coating. At this point it should be noted that the two conveying lines 66-44-77 can be fed with lids 8 of different dimensions and/or configurations, for reasons which will be apparent hereinafter. Said conveyors 66, 44 and 77 are driven at the same speed by a geared motor unit 11 (see Figures IB, 3) with an incorporated variable speed gear, the drive being via a toothed belt 12 which rotates the shaft 100. The shaft 120 (Figure 3) which drives said toothed belt 12 drives a further toothed belt 13 which moves the chain 50 at the same speed as the conveyors 66, 44 and 77. According to the invention, and as can be seen in all the accompanying figures, the lids 8 traverse while resting in a flat position with their turned edge 88 having its concavity facing upwards (Figure 4), the liquid surface of the bath 4 in the tank 3 being maintained above the upper (inner) face of the lids 8. The outward and return branches of the conveyor 44 are rectilinear, the former entering and leaving the tank 3 through corresponding weired apertures 14 (see Figure 2) provided in the end walls 140 of said tank 3. The liquid 4 continuously discharges from said apertures and is collected in the underlying tank 7, from which it is again fed to the tank 3 by a pumping unit (not shown) which is arranged to maintain a level of liquid 4 within the tank 3 such as to ensure that the lids 8 and the ends of the appendices 55 forming the anode are completely immersed. Suitable guide walls 15 direct towards the tank 7 the liquid 4 leaving the apertures 14, the liquid falling from the return branch of the conveyor 44 and the liquid removed from the recoated lids 8 by air nozzles 19 shown in Figure IB. It should be noted that the continuous recircυlation and remixing of the liquid 4 eliminate any inconvenient dilution of the coating particles in the lid transit region. As can be seen in Figure 2, said apertures 14 are conveniently shaped both to allow passage of the elements described hereinafter and to limit the discharge of liquid 4. Said elements comprise for each individual conveyor 44 a longitudinal metal plate 16 for the support and sliding of the conveyor outward branch and two opposing lateral longitudinal metal guides 17. Said plate 16 and guides 17 are suitably electrically insulated from the surrounding metal machine components and are preferably constructed of stainless steel or surface treated/hardened steel. Specifically, as shown in Figure 2, that guide 17 of each pair which is close to the longitudinal axis of the tank 3 is securely fixed to the tank base, whereas the opposite guide 17 of the same pair and said plate 16 can be adjusted transversely to said base. This is achieved by respective clamping devices 171 and 161 which are received in corresponding transverse slots 170 a d 160. In this manner the distance between the guides 17 can be matched to the diameter (or width) of the lids 8 in transit, and the plate 16 can be substantially aligned with the central region of the lids 8. Suitable means are provided for transversely adjusting the pulley units 9, 91 and 10, 101, and in addition the tank 3 is provided with a series of interchangeable end walls 140 to allow said adjustments. In addition, the two guides 17 associated with each conveyor 44 comprise two longitudinal grooves 18 with their openings facing each other, to receive the corresponding opposing portions of the turned edge 88 of the lid 8. Said two grooves 18 (see Figure 2) provide two lower narrow horizontal support surfaces which maintain the lids 8 raised from the conveyor 44, two opposing vertical surfaces to prevent derailing of the lids 8 pushed by the pushers 40, and two upper horizontal surfaces to prevent untimely rocking of the lids 8 in those vertical planes transverse or parallel to their direction of advancement. With reference to those two guides 17 close to the vertical plane of longitudinal symmetry of the tank 3, within this latter said surfaces present an aperture for the passage of the constituent appendices 55 of the anode. In the case illustrated, said two groups of anodes 55 are in the form of perpendicular appendices of a chain 50 of stainless steel or another suitable hard good conducting material, however a narrow steel tape could also be used. Each chain 50 passes about sprockets 20, not electrically insulated, and slides in a suitable guide 80 of insulating material which in the lower branch ensures that the appendices or anodes 55 make electrical contact with the upper surface of the lids 8. Said contact is clearly shown in Figure 4, which also shows the coating film 110 which forms on the anode 55 as it traverses the bath 4, and which is subsequently removed as stated hereinafter. With reference to Figure 3, in a position opposite said wheels 80 there is fixed to the corresponding guide 17 a flat band 116 extending for the total length and projecting towards the underlying lids 8 in transit, to form together with the plate 16 the negative electrode or cathode of the bath 4. Said cathode 16, 116 is suitably connected to an electrical power supply, indicated by 21 in Figure IB, which applies to the bath a d.c. voltage adjustable for example from 100 to 500 volts depending on the specific parameters governing the electrophoretic process underway. The same power supply 21 is also connected to one of the pairs of sprockets 20 by suitable contactors 22, indicated schematically in the upper part of Figure 3 and consisting for example of graphite brushes, slide shoes or the like. The chain 50, which is not immersed in the bath, is always clean, whereas the ends of the electrodes 55 are cleaned by immersion in the overlying bath 6 during the return travel of the chain. Two vessels 25 and 26 (Figure 1A) are provided for the cleaning liquid 6 and the concentrated coating material respectively, there being also provided, although not shown, suitable apparatus for measuring the bath temperature, conductivity and pH, for temperature control of this latter and of the cleaning liquid tank 5, for adjusting the power supply 21, etc. Finally, indicator lamps 28 are provided, plus a transparent panel 27 (Figure 3) for visually monitoring the essential components contained in the chamber 2.

Claims: PATENT CLAIMS 1. A machine for the electrophoretic coating of thin-walled metal objects, such as tinplate lids, of the type comprising a containing tank (3) for the electrophoretic bath (4), a cathode immersed in the bath and an anode partially immersed in the bath and brought into contact with the objects to be coated, and a conveying line for advancing the objects immersed in the bath, the anode consisting of a vertical endless chain (50), the lower branch of which runs above the liquid surface of the electrophoretic bath, and from which there extend electrodes in the form of flexible appendices (55) to be partially immersed in said bath in order to come into contact with the objects to be coated. 2. A machine as claimed in claim 1, characterised in that above the electrophoretic liquid bath (4) there is positioned a bath (6) of cleaning liquid into which the electrodes (55) are partially immersed during the return travel of the chain (50) in order to remove from said electrodes (55) most of the coating film (110) present on them. 3. A machine as claimed in claim 2, characterised in that said anode is made to move at the same speed as the lids in transit. 4. A machine as claimed in claim 2, characterised in that said at least one conveying line comprises an endless element (44), such as a chain, provided with a plurality of equidistant projecting elements (40) arranged to push said lids between two opposing longitudinal guides (17), which are situated on one and the other side of said line and are provided with respective seats (18) which receive the two opposing marginal portions of said turned edge. 5. A machine as claimed in the preceding claims, characterised in that said two guides (17) of said at least one conveying line are at an adjustable distance apart. 6. A machine as claimed in the preceding claims, characterised in that said conveying element (44) has its outward and return branches rectilinear and positioned respectively above and below the base of said tank, said outward branch passing through the end walls (140) of said tank via respective weired apertures (14). 7. A machine as claimed in claim 6, characterised in that said end walls (140) are interchangeable, said weired apertures lying above a tank (7) for collecting the liquid which falls, this latter tank being provided with recirculation means for feeding the liquid back into the overlying tank (3). 8. A machine as claimed in claim 4, characterised in that on the shafts (90), (100) about which said conveying element (44) passes there reverse a conveyor (66) on which the lids to be recoated arrive and a conveyor (77) on which the recoated lids leave, each conveyor being provided with equidistant pusher elements (60) and (70) having the same pitch as those (40) of the element (44), with the leaving conveyor (77) there being associated blower means (19) to remove from the lids (8) in transit the liquid retained by them. 9. A machine as claimed in claim 1, characterised in that said cathode comprises two plates (16), (116) positioned respectively below and above the outward branch of said conveying element (44).

Inventors: BALDASSARI GIANFRANCO, BALDASSARI, GIANFRANCO
Applicants: BALDASSARI GIANFRANCO, CORIMA SARL, BALDASSARI, GIANFRANCO, CO.RI.MA SOCIETA' A RESPONSABILITA' LIMITATA
Application year: 1994
Main classification: C25D13
IPC codes: C25D13